Electron-doping evolution of the low-energy spin excitations in the iron arsenide superconductorBaFe2−xNixAs2

Abstract

We use elastic and inelastic neutron scattering to systematically investigate the evolution of the low-energy spin excitations of the iron arsenide superconductor BaFe2−xNixAs2 as a function of nickel doping x. In the undoped state, BaFe2As2 exhibits a tetragonal-to-orthorhombic structural phase transition and simultaneously develops a collinear antiferromagnetic (AF) order below TN = 143 K. Upon electron-doping of x = 0.075 to induce bulk superconductivity with Tc = 12.3 K, the AF ordering temperature reduces to TN ≈ 58 K. We show that the appearance of bulk superconductivity in BaFe1.925Ni0.075As2 coincides with a dispersive neutron spin resonance in the spin excitation spectra, and a reduction in the static ordered moment. For optimally doped BaFe1.9Ni0.1As2 (Tc = 20 K) and overdoped BaFe1.85Ni0.15As2 (Tc = 15 K) superconductors, the static AF long-range order is completely suppressed and the spin excitation spectra are dominated by a resonance and spin-gap at lower energies. We determine the electron-doping dependence of the neutron spin resonance and spin gap energies, and demonstrate that the three-dimensional nature of the resonance survives into the overdoped regime. If spin excitations are important for superconductivity, these results would suggest that the three-dimensional character of the electronic superconducting gaps are prevalent throughout the phase diagram, and may be critical for superconductivity in these materials.

Polarized neutron scattering studies of magnetic excitations in electron-overdoped superconducting BaFe1.85Ni0.15As

“…In a previous unpolarized neutron scattering experiment performed on BaFe 1.85 Ni 0.15 As 2 , a neutron spin resonance was observed near E = 6 meV in the superconducting state, found by taking a temperature difference between constant-Q scans at (0.5, 0.5, 1) r.l.u. 16 . Before determining the possible magnetic anisotropy from neutron polarization analysis, we note from Eq.…”

“…Assuming the background scattering has no temperature dependence across T c , the temperature difference data of σ SF α + σ NSF α should recover unpolarized neutron scattering results 16 . Figures 3(a) and 3(b) show the sum of the raw data σ SF α + σ NSF α above and below T c , respectively for α = x, y and z.…”

“…Near the optimally electron-doped superconductor BaFe 2−x Ni x As 2 at x = 0.1 (T c = 20 K), the static AF order is suppressed 12 . However, short-range spin excitations persist and couple directly to superconductivity via a collective magnetic excitation termed the neutron spin resonance [12][13][14][15][16][17] . As a function of Ni-doping, the energy of the resonance is associated with both the superconducting electronic gap ∆ and k B T c , thus indicating its direct connection with superconductivity 18 .…”

“…Although the resonance appears to be a common feature amongst different classes of unconventional superconductors including high-T c copper oxides [19][20][21][22][23] , heavy Fermions 24,25 , and iron-based materials [12][13][14][15][16][17][26][27][28] , much remains unknown about its microscopic origin. Assuming that the resonance is a spin-1 singlet-to-triplet excitation of the Cooper pairs 29 , it should be possible to split it into three peaks under the influence of a magnetic field via the Zeeman effect by an amount ∆E = ±gµ B B, where g = 2 is the Lande factor and B is the magnitude of the field [30][31][32][33] .…”

Anisotropic neutron spin resonance in superconductingBaFe1.9Ni0.1As2

“…[1][2][3][4] Since its initial discovery in optimal hole-doped YBa 2 Cu 3 O 6+x , [1][2][3][4] the resonance has been found in electron-doped cuprates, 5 heavy fermion, 6,7 and iron arsenide superconductors. [8][9][10][11][12][13] Below the superconducting transition temperature T c , the intensity of the resonance increases like the superconducting order parameter and its energy scales with T c . 5 Although the resonance appears to be a ubiquitous property of unconventional superconductors, [1][2][3][4][5][6][7][8][9][10][11][12] its microscopic origin and relationship with superconductivity are still debated.…”